Chemically Inert Pressure Sensitive Adhesive Having Improved Adhesion, Process For It&#39;s Manufacture, And Use Thereof

ABSTRACT

Chemically inert pressure sensitive adhesives having improved adhesion to polar and semipolar substrates, as well as to the manufacture and use thereof, especially for manufacturing pressure sensitive adhesive tapes possessing high initial adhesion (tack) on polar and semipolar substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of InternationalApplication No. PCT/EP03/012116, filed on Oct. 31, 2003, which claimspriority of German application number 102 52 978.7, filed on Nov. 14,2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pressure sensitive adhesives which areresistant to chemicals and process additives and at the same timeexhibit improved adhesion to polar and semipolar substrates, especiallyon glass.

More particularly, the invention relates to the manufacture ofchemically inert pressure sensitive adhesives and their use, especiallyfor the production of pressure sensitive adhesive tapes with highinitial adhesion (tack) on polar and semipolar substrates such as glass,metal, polycarbonate and similar materials.

2. Description of the Prior Art

The tack of a pressure sensitive adhesive tape is dependent on thesubstrate to which the pressure sensitive adhesive tape is applied. Forsubstrates with polar surfaces, such as glass or metal, a value of about25 to 30 N/25 mm, as can be achieved with the at present commonly usedpure acrylate-based pressure sensitive adhesive tapes, is regarded ashigh. With substrates that have nonpolar surfaces, such as polyethyleneor polypropylene, values of more than 15 N/25 mm are already consideredhigh. The pure acrylate-based pressure sensitive adhesive tapes whichare commonly used today have a tack on substrates with nonpolar surfacesthat is in the range between 5 and 12 N/25 mm.

Pressure sensitive adhesive tapes having high tack on polar substratesare widely utilized in industrial processes. The pressure sensitiveadhesives utilized for such pressure sensitive adhesive tapes usuallyconsist of polyacrylates or mixtures of polyacrylates with resins(tackifiers). The polyacrylates are produced by free-radicalpolymerisation of mixtures of monomeric acrylates, for example esters ofacrylic acid, methyl acrylic acid, ethyl acrylic acid, butyl acrylicacid and the like, or of the acids themselves. Polymerization isperformed in solvents by thermal activation, or in mass by activationvia high-energy irradiation (UV or EB curing).

To achieve a high initial adhesion (tack), acrylic acid may be used. Thehigh tack is here brought about by the interaction of the strongly polarcarboxyl group of the acrylic acid with the polar or semipolarsubstrates (dipole-dipole-interaction, formation of hydrogen bridges).

The use of acrylic acid to achieve high initial adhesion to polarsurfaces is, however, connected with serious disadvantages. Because ofthe strongly polar character of acrylic acid, these adhesives show highaffinity to polar and semipolar solvents. Long-lasting exposure to suchadhesives to polar or semipolar solvents as a rule results in failure ofthe adhesive joint due to swelling (cohesive failure) or detachment ofthe adhesive from the surface of the substrate as a result of migrationof the solvent in the barrier layer between adhesive and the substratesurface (adhesive failure).

A further disadvantage of pressure-sensitive adhesives containingacrylic acid is the strong reactivity of acrylic acid to reactivechemical compounds. Particularly in the manufacture of compound glassand compound glass products wherein cast resins based onisocyanate-containing polymers are utilized, there occurs a reaction ofthe isocyanate groups with the acrylic acid in the pressure sensitiveadhesive. This reaction manifests itself in an undesired formation ofbubbles due to carbon dioxide formation caused by the followingreaction: R₁—NCO+R₂—COOH→R₁—NH—CO—R₂+CO₂. This bubble formation cannotonly affect the functional capability of the constructional elements,but is also not desired for aesthetic reasons, above all in adhesivejoints at visible sites.

It was therefore the task of the present invention to provide a pressuresensitive adhesive which has high tack while simultaneously is inert tochemical influences and moisture. At the same time the adhesive shouldnot show a visible reaction on contact with isocyanate-containingcompounds.

SUMMARY OF THE PRESENT INVENTION

The task according to the present invention is solved by pressuresensitive adhesives based on copolymerisates of methacrylates andacrylates. The desired high tack of the inventive pressure sensitiveadhesives is achieved by polar acrylates such as 2-hydroxyethylacrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, PEGacrylates, 2-aminoethyl acrylate, 3-aminopropyl acrylate, 4-aminobutylacrylate, or by methacrylates corresponding to the preceding compounds.Important for achieving a high tack are polar groups in the acrylateswhich are capable of forming hydrogen bridges to the substrates. Theseare, in particular, hydroxy, amino and thio groups, as well as, to aminor extent, urea groups, urethane groups and imino groups, that is,generally, acrylates having at least one Tserevitinov hydrogen.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

It should be appreciated that the term “Tserevitinov hydrogen” means theso-called active hydrogen which can be determined, using a method foundby Tschugaeff and Tserevitinov, by conversion with methyl magnesiumiodide in butyl ether or other ethers. Belonging to the Tserevitinovhydrogens is the active N—, O— or S-bound active hydrogen of carboxy,hydroxy, amino and imino as well as thiol groups, but also the activehydrogen of H-acidic hydrocarbons, which in some cases can be quantifiedby the methods of Tschugaeff and Tserevitinov.

According to the present invention, polar(meth)acrylates which do notcontain carboxyl groups, but which are selected from the group of thehydroxy(meth)acrylates, may be employed.

The necessary consistency of the pressure sensitive adhesives in termsof softness and processibility is, according to the invention, achievedby using nonpolar acrylates and/or methacrylates. Suitable for thispurpose are, for example, esterification products of acrylic acid andmonohydric alcohols, as well as the corresponding products which areformed when using methacrylic acid. According to the invention, theproducts of C₄-C₁₅ alcohols may be employed, such as the conversionproducts of C₆-C₁₀ alcohols.

To achieve an inner strength of the pressure sensitive adhesives(cohesion), a crosslinking agent can be added to the acrylate and/ormethacrylate mixtures during polymerisation. The crosslinking agents areon the one hand acrylates or methacrylates of the dihydric, trihydric orhigher valent alcohols, or analogous vinyl ethers (in the following alsodesignated as difunctional, trifunctional and higher functional(meth)acrylates) or mixtures thereof. On the other hand, the polymerscan also be crosslinked subsequent to polymerisation, including thepolar acrylates, by means of isocyanates, epoxides, aziridines, metalcomplexes or other multifunctional compounds reacting with Tserevitinovhydrogen, or by means of mixtures thereof.

As a matter of course, mixtures of (meth)acrylate-containingcrosslinking agents with compounds that are reactive to H-acidiccompounds are also possible.

The degree and type of crosslinking in the production of the inventivepolymer composition is determined by the degree of softness and innerstrength of the adhesive polymers that is required for their laterapplication, as well as by further parameters such as heat resistance,low-temperature resistance, tack and the like.

The inventive adhesives can be prepared from:

-   -   (a) 1 to 50%-wt of polar(meth)acrylates with a Tserevitinov        hydrogen,    -   (b) 50 to 99%-wt of apolar(meth)acrylates,    -   (c) up to 10%-wt of a bi-, tri- or higher functional        (meth)acrylates or of a (poly)functional compound which is        reactive to Tserevitinov hydrogens, the weight percentages        indicated under a) to c) adding up to 100%-wt,    -   (d) 0.05 to 5%-wt of an initiator, relative to the sum of the        components of a) to c), the weight percentages indicated        under a) to d) adding up to 100%-wt, and    -   (e) up to 90%-wt of a liquid, chemically inert medium, relative        to the components a) to d).

It should be appreciated that “polar(meth)acrylates with Tserevitinov”hydrogen according to a) means reaction products of acrylic ormethacrylic acid with di-, tri- or higher functional alcohols as well astheir ethoxylation products, with terminal hydroxy, amino, thio orheterocyclic groups containing at least one active hydrogen. As a matterof course, for the purposes of the invention it is also possible toutilize mixtures of these compounds.

Table 1 lists some examples of technically important compounds.

Table 1: Examples of Technically Significant (Meth)Acrylates with ActiveHydrogen

R₁ = CH₃, H R₂ = C₂H₄, C₃H₆, C₄H₈ Hydroxyalkyl (meth)acrylates

R₁ = CH₃, H R₂ = C₂H₄, C₃H₆, C₄H₈ R₃ = H, CH₃, C₂H₅, C₃H₇, C₄H₉Aminoalkyl (meth)acrylates

R₁ = CH₃, H R₂ = C₂H₄, C₃H₆, C₄H₈ Thioalkyl (meth)acrylate

R₁ = CH₃, H R₂ = CH₃, H n = 1-15 PEG/PPG-(meth)acrylates

It should also be appreciated that apolar(meth)acrylates according to b)are understood to be conversion products of monofunctional alcohols oramines with acrylic acid or methacrylic acid, as well as mixturesthereof.

Technically significant representatives of this class aremethyl(meth)acrylate, ethyl acrylate, butyl acrylate,hexyl(meth)acrylate, isooctyl acrylate, 2-ethylhexyl acrylate,isodecyl(meth)acrylate, isobomyl (meth)acrylate, alkyl(meth)acrylamides.But still other monomers which are accessible to free-radicalpolymerisation, e.g. vinyl compounds, can be added in minor amounts tothe inventive (meth)acrylate mixtures, should this be necessitated byspecial demands put on the adhesive polymer.

A di-, tri- or higher functional (meth)acrylate according to c) isunderstood to mean compounds obtained by the reaction of polyhydricalcohols with (meth)acrylic acid. Some technically important compoundsof this class are listed in Table 2.

Table 2: Examples of Technically Relevant Higher Functional(Meth)Acrylate-Containing Compounds

R₁ = H, CH₃ R₂ = C₂H₄, C₃H₆, C₄H₈, C₆H₁₂ Alkyl di(meth)acrylates

R₁ = H, CH₃ R₂ = H, CH₃ n = 2-15 Divinyl ethers

R₁ = H, CH₃ R₂ = H, CH₃ n = 2-15 Alkoxyl di(meth)acrylates

R1 = H, CH3 R2 = H, CH3 Epoxidized bisphenol di(meth)acrylates

R = H, CH₃ Trimethylol propane tri(meth)acrylates

R₁ = H, CH₃ R₂ = H, CH₃ Alkoxylated trimethylol propanetri(meth)acrylates

Considered as belonging to the class of “higher valent(meth)acrylate-containing compounds” are furthermore, for the purposesof the afore-mentioned definition of c), the compounds of the family of(meth)acrylated polyesters as well as of the (meth)acrylatedpolyurethanes. The (meth)acrylated polyesters are the reaction productsof OH-terminated polyester polyols, mostly of oligomeric origin, with(meth)acrylic acid, or the products of the reaction of carboxylgroups-containing polyester polyols, likewise mostly in oligomeric form,with OH groups-containing (meth)acrylates. The (meth)acrylatedpolyurethanes are conversion products of amine-terminated orhydroxyl-terminated (meth)acrylates with di-, tri- or polyisocyanates(for example, 1,6-hexamethylene diisocyanate, 2,4- or 2,6-toluylenediisocyanate, isophorone diisocyanate), the urea-, biuret- oralophanate-containing derivatives thereof, or the dimeric and trimericcompounds which originate from the monomeric compounds.

It should be further appreciated that a compound reactive toTserevitinov hydrogens according to c) means epoxides, isocyanates,cyanates, aziridines, metal chelates or other compounds corresponding tothe state of the art which are capable of bonding chemically toC—H-acidic compounds.

It should be appreciated that an initiator according to d) meanscompounds which, under action of thermal energy or high-energyradiation, are capable of decomposing into radicals and of starting afree-radical polymerisation. Examples of such initiators are azocompounds, peroxide compounds, aromatic α-hydroxyketones, aryl phosphorcompounds, and similar radical initiators.

It should also be appreciated that a liquid, chemically inert mediumaccording to e) means a liquid compound which is capable of detachingthe polymer formed, or to emulsify or disperse it. Belonging to thisgroup are esters, ketones, alcohols, hydrocarbons, cycloaliphatics,aromatics, water, or mixtures of these compounds.

The preparation of the inventive polymer compounds can be performed bothin organic solvents (solution polymerisation) as well as in water(emulsion polymerisation) but also free of solvents by UV light orelectron beam. Here, the manufacture and processing of the inventivepolymers takes place in a known manner.

In the solvent-containing processes, the polymer is produced in a knownmanner in a liquid, chemically inert medium (component according to e))from (meth)acrylates with Tserevitinov hydrogen, apolar (meth)acrylatesand an initiator (the components according to a), b), d)) (solutionpolymerisation). Here, solutions of adhesive polymers having a typicalsolids content of 10-70%-wt are obtained. This solution is reacted witha compound according to c) which is reactive to Tserevitinov hydrogens.Subsequently, the solution can be processed on a coating plant with adownstream drying channel according to the state of the art, so as toyield a web-like adhesive or sealant.

In the water-based processes, (meth)acrylates with Tserevitinovhydrogen, apolar(meth)acrylates and an initiator (the componentsaccording to a) to c)) are emulsified with water. For this purpose, itmay be expedient to use additional dispersing or emulsifying agentsknown to those skilled in the art. The reaction mixture is converted ina known manner to a polymer dispersion or emulsion having a solidscontent between 10 and 70%-wt. This dispersion can then be processed ina manner analogous to the solvent-based polymers to form web-likesealants and adhesives.

In the solvent-free processes a polymer syrup is prepared fromcomponents a) to c) in a known manner. Utilized as components c) arepreferably di-, tri- or higher functional (meth)acrylates. The polymersyrup can subsequently be processed to adhesives and sealants on acoating apparatus with a downstream curing device, by means ofhigh-energy radiation.

The inventive polymer composition can be employed as adhesive orsealant. Preferably, the inventive polymer composition is furtherprocessed to pressure sensitive adhesive tapes. Using the pressuresensitive adhesive tapes prepared with the aid of the inventivepolymers, an initial adhesion of up to 50 N/25 mm on substrates with apolar surface, and of up to 20 N/25 mm on substrates with a nonpolarsurface was measured.

EXAMPLES 1. Preparation of a Solvent-Based, Chemically Inert Adhesive

250 parts by weight of isodecyl methacrylate and 250 parts by weight of2-hydroxyethyl acrylate are dissolved in 450 parts by weight of ethylacetate and, under introduction of nitrogen and stirring, heated tillboiling. Subsequently, 5 parts by weight of azoisobutyronitrile,dissolved in 50 parts by weight of ethyl acetate, is metered theretoover a period of 1 hour. After the initiator has been added, thereaction mixture is left for 5 hours, under back flow. After cooling, anadhesive solution is obtained which has a solids content of 50%-wt. Theresidual monomer content amounts to maximally 2.5%-wt. 10 parts byweight of a 10% solution of titanium chelate in isopropanol is added tothe adhesive solution. The solution is spread to form a film of 0.2 mmin thickness. After evaporation of the solvent, the film is dried at 90°C. for 10 min. A pressure-sensitive film is obtained.

2. Preparation of a Solvent-Free, Chemically Inert Adhesive

250 parts by weight of isodecyl methacrylate and 250 parts by weight of2-hydroxyethyl acrylate are mixed with 5 parts by weight of IRGACURE®819 (CIBA Spezialitatenchemie) and 2.5 parts by weight of tripropyleneglycol diacrylate. After knife-coating a 0.5 mm thick layer onto acarrier film (PET), the former is cured in nitrogen atmosphere within 1minute by means of a UV lamp (80 W/cm). A pressure sensitive adhesivefilm is obtained. Conversion amounts to 98%.

3. Preparation of a Solvent-Based Adhesive

250 parts by weight of isodecyl methacrylate, 150 parts by weight of2-ethylhexyl acrylate, 50 parts by weight of 2-hydroxyethyl acrylate and50 parts by weight of acrylic acid are dissolved in 450 parts by weightof ethyl acetate. The polymerisation and processing is performed in amanner analogous to Example 1. The solids content of the adhesivesolution amounts to 50%-wt, the residual monomer content is maximally2.5%-wt. After drying, a pressure sensitive adhesive film is obtained.

4. Preparation of a Solvent-Free Adhesive

250 parts by weight of isodecyl methacrylate, 200 parts by weight of2-hydroxyethyl acrylate, 50 parts by weight of acrylic acid, 5 parts byweight of IRGACURE®819 (CIBA Spezialitätenchemie), and 2.5 parts byweight of tripropylene glycol diacrylate are mixed. Processing isperformed in a manner analogous to Example 2. A pressure sensitiveadhesive film is obtained. Conversion amounts to at least 98%.

5. Examination of the Resistance of the Adhesives to Reactive CastResins

The adhesive films were applied to an aluminium sheet and covered attheir margins with a layer of isocyanate (HDI), so that the isocyanatewas in contact with both the adhesive and the aluminium substrate. Thesamples were observed for 24 hours. Example CO₂-formation Adhesion toglass Adhesion to Al 1 none good good 2 none good good 3 after 10 mingood good (Comparison) 4 after 10 min good good (Comparison)

The adhesives according to the invention are characterized by their highresistance to reactive substances. At the same time adhesion to polarsubstrates is not detrimentally affected compared to conventional(acrylic acid-containing) adhesives.

What has been described above are preferred aspects of the presentinvention. It is of course not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe present invention, but one of ordinary skill in the art willrecognize that many further combinations and permutations of the presentinvention are possible. Accordingly, the present invention is intendedto embrace all such alterations, combinations, modifications, andvariations that fall within the spirit and scope of the appended claims.

1. A polymer composition which can be produced by polymerisation of a) 1to 50%-wt of polar(meth)acrylates with Tserevitinov hydrogen, saidpolymer(meth)acrylates being selected from the group consisting of2-hydroxyethyl(meth)acrylate, 3-hydroxypropyl(meth)acrylate,4-hydroxybutyl(meth)acrylate, PEG (meth)acrylates, PPG (meth)acrylates,2-aminoethyl(meth)acrylate, 3-aminopropyl (meth)acrylate,4-aminobutyl(meth)acrylate, reaction products of acrylic or methacrylicacid with bi-, tri- or higher functional alcohols, and ethoxylation,propoxylation and butoxylation products of acrylic or methacrylic acidwith terminal hydroxy, amino, urethane or thio groups containing atleast one active hydrogen; b) 50 to 99%-wt of apolar(meth)acrylates; c)in the presence of a bi-, tri- or higher functional (meth)acrylate, orof a (poly)functional compound which is reactive to Tserevitinovhydrogens and is selected from the group consisting of mono-, bi- andpolyepoxides, mono-, bi- and polyaziridines and melamine and itsderivatives, or of a mixture of two or more of the aforementionedcompounds, the weight percentages indicated under a) to c) adding up to100%-wt.; d) with addition of 0.05 to 5%-wt. of an initiator, relativeto the sum of the components of a) to c); and e) up to 90%-wt. of aliquid, chemically inert medium, relative to the solid matter content ofthe sum of the components of a) to d).
 2. The polymer compositionaccording to claim 1, wherein said polar(meth)acrylates do not containcarboxyl groups.
 3. The polymer composition according to claim 1,wherein said polar(meth)acrylates are hydroxy(meth)acrylates.
 4. Thepolymer composition according to claim 1, wherein saidpolar(meth)acrylates are amino(meth)acrylates.
 5. The polymercomposition according to claim
 1. wherein said apolar(meth)acrylates areesterification products of acrylic acid or methacrylic acid withmonovalent alcohols or amines.
 6. The polymer composition according toclaim 5, wherein said apolar(meth)acrylates are alkyl(meth)acrylamides.7. The polymer composition according to claim 6, wherein saidapolar(meth)acrylates are esterification products of acrylic acid ormethacrylic acid with monohydric alcohols having 6 to 15 C atoms.
 8. Thepolymer composition according to claim 7, wherein saidapolar(meth)acrylates are selected from the group consisting ofmethyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate,hexyl(meth)acrylate, isooctyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,isodecyl(meth)acrylate and isobomyl(meth)acrylate.
 9. The polymercomposition according to claim 1, wherein said di-, tri- or higherfunctional (meth)acrylates are selected from the group consisting of theconversion products of (meth)acrylic acid with diols, triols or polyols,the analogous vinyl ethers or mixtures thereof, (meth)acrylatedpolyesters and (meth)acrylated polyurethanes.
 10. The polymercomposition according to claim 9, wherein said (meth)acrylatedpolyesters are conversion products of OH-terminated polyester polyolswith (meth)acrylic acid or reaction products of carboxylgroups-containing polyester polyols with hydroxyl groups-containing(meth)acrylates.
 11. The polymer composition according to claim 9,wherein said (meth)acrylated polyurethanes are conversion products ofamine- or hydroxyl-terminated (meth)acrylates with diisocyanates orpolyisocyanates.
 12. The polymer composition according to claim 1,wherein said polymer composition contains a further compound which isreactive to Tserevitinov hydrogen and which is selected from the groupconsisting of mono-, di- and polyisocyanates.
 13. A process for theproduction of a polymer composition according to claim 1, said processcomprising a polymerisation reaction which is performed free of solvent,or in a liquid selected from the group consisting of water and anorganic, inert solvent.
 14. Use of the polymer composition according toclaim 1 as an adhesive or as a sealant.
 15. Use of the polymercomposition according to claim 1 for the production of pressuresensitive adhesive tapes.
 16. The polymer composition according to claim7, wherein said apolar (meth)acrylates are esterification products ofacrylic acid or methacrylic acid with monohydric alcohols having 6 to 10C atoms.